Atomic line filters are a class of optical filters which have acceptance bandwidths on the order of 0.001 nm. In one prior art type of ALF, broadband light containing narrowband signal light is passed through a first color glass filter which cuts off light at wavelengths below a threshold value. The signal and remaining noise light enter an atomic vapor that only absorbs the signal light within the atom's 0.001 nm acceptance bandwidth thereby exciting those absorbing atoms to an intermediate energy level. A pump beam further excites those atoms to a second, higher energy level that then decays through various processes including fluorescence, to the ground state of the atom. The emitted fluorescence occurs at wavelengths below the threshold value of the first color glass filter. A second color glass filter then cuts off any wavelengths above the threshold which effectively permits passage of only the emitted narrowband fluorescence. In effect, the incoming signal has been internally shifted in wavelength by the atomic vapor, which then allows the use of two overlapping color glass filters to block any background radiation.
Another type of prior art atomic line filter takes advantage of either the Faraday effect or the Voigt effect where an atomic vapor in a magnetic field produces polarization rotation in order to pass a narrow spectral band of light through two crossed polarizers. These filters are known respectively as Faraday filters or Voigt filters. An important use for these filters is to block background light so that a beacon laser beam can be detected by a wide field-of-view detector.
Operational principles of Faraday filters can be understood by reference to FIGS. 7A-C. Crossed polarizers 90 and 91 serve to block out background light with a rejection ratio better than 10−5. Because these polarizers only work over a limited wavelength region in the infrared, a broad band interference filter may be used in conjunction with the Faraday filter. Between the polarizers, an atomic vapor (which in many of these filters is cesium or rubidium) in a magnetic field axially aligned with the path of the beam, rotates the polarization of the laser signal by 90°, while leaving background light at other wavelengths unrotated, and thus blocked by the polarizers.
In the case of the Faraday filter the magnetic field is applied in the direction of the signal beam, and in the case of the Voigt filter the magnetic field is applied perpendicular to the signal beam direction and at 45 degrees to the direction of each of the two cross polarizers.
Prior art atomic line filters patents issued to co-workers of applicant includes U.S. Pat. Nos. 4,983,844; 5,267,010; 5,502,558; 5,731,585 and 6,151,340 each of which are incorporated herein by reference. The '844 patent discloses a fast atomic line filter which utilizes a pump laser and a high voltage potential to produce ion pairs from atoms excited by photons with wavelengths corresponding to a resonant frequency. The other patents describe applications of Faraday and/or Voigt filters.
One problem with atomic line filters such as those referred to above is that their operation depends on the existence of a good sharp resonant absorption line near the spectral range to be filtered. Many of these sharp resonant absorption lines are characteristic of atomic vapors and the filters described in the above referenced patents utilize alkali metals such as cesium and rubidium to produce these metal vapors. These metals are preferred because their vapors may be produced at relatively low temperatures. However, good absorption lines from these alkali metal vapors are generally in the visible and the near visible spectral region such as 780 nm and 852 nm.
Many optical symptoms operate at wavelengths substantially longer than the visible and near visible. A good example is light with wavelengths in the range of 1.5 micron. For example, fiber optic communication is typically at wavelengths in the range of about 1.2 micron to about 1.65 microns (see FIG. 5). Typical long haul filter optics operate within a C or L band. (C band is 1520 nm to 1570 nm, and L band is 1570 nm to 1620 nm). Shorter range fiber optics requiring higher quality fiber optics may operate within an s band (1.31-1.48 microns). Also, there is a need for filters at even longer wavelength such as about 1.5 to 5 microns for laser tracking and for free space laser communications. Applicants have searched for good resonance lines in the alkali metals at these wavelengths without success.